Chapter A: General Diagnosis
13
are available in many styles and designs. Current mod-
els are designed to sample and analyze either four or five
gasses present in the exhaust from the vehicle. The
newest models are designed for five-gas detection and
normally provide digital and/or printed results of each
test. Either piece of equipment is generally suitable for
diagnosing basic fuel system abnormalities and drive-
ability problems. The five-gas units are required in many
jurisdictions to permit the technician to verify emissions
compliance of a vehicle.
Five-Gas Analyzers
Five-gas analyzers measure the parts per million (ppm)
of hydrocarbons (HC), the percentage of carbon monox-
ide (CO), the percentage of oxygen (O2), the percentage
of carbon dioxide (CO2), and the percentage of oxides
of nitrogen (NOx). Most properly tuned computer-con-
trolled vehicles will produce about 50 ppm of HC, less
than 0.5 percent CO, 1.0 to 2.0 percent O2, and 13.8 to
15.0 percent CO2.
Four-Gas Analyzers
Four-gas analyzers measure HC, CO, CO2, and O2.
They do not provide data as to the levels of NOx in the
exhaust.
Diagnosing Exhaust Gasses
To assist with the discussion that follows, refer to the di-
agnostic chart for problems that may cause abnormal
readings, figure 1-15.
For an accurate analysis of fuel combustion on catalyt-
ic converter-equipped vehicles, prevent the air injection
system from supplying oxygen into the exhaust stream.
This decreases the amount of O2 at the tailpipe, and
the efficiency of the converter. The air injection system
may be disabled by several means. On some vehicles,
disconnect the air injection pump or plugging the pulse
air injection system is effective. For others, the probe of
the analyzer can be connected to a port installed up-
stream of the catalytic converter or to the exhaust
opening for the EGR valve. Next, make sure that the
engine is at operating temperature, in closed loop, and
the HO2S is transmitting a variable signal. Sample the
exhaust gases both at idle and at 2,500 RPM. If a dy-
namometer is used, test under simulated highway load
conditions as described by the manufacturer.
Abnormal HC and CO Readings
High HC levels indicate unburned fuel in the exhaust
caused by incomplete combustion. The source of high
HC emissions can often be traced to the ignition sys-
tem, but mechanical or fuel system problems also can
increase HC emissions, figure 1-16. High levels of HC
emissions result from:
• Advanced ignition timing
• Ignition misfire from defective spark plug wires or
fouled spark plugs
• An excessively rich or lean air-fuel mixture
• Leaking vacuum hoses, vacuum controls, or seals
• Low engine compression
• Defective valves, valve guides, valve springs,
lifters, camshaft, or incorrect valve lash
• Defective rings, pistons, or cylinder walls
• Clogged fuel injectors causing a lean misfire
Fig. 1-13.
Typical breakout box.
Fig. 1-14.
Typical exhaust gas analyzer.
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